Sulfur is one of the most widely distributed chemical elements. It occurs as a sulfide or as a sulfate in ores of metals. It is also combined with organic and inorganic elements in fossil fuels. Hence, in the extraction of metals from ores or in the combustion of fuels sulfur dioxide is often one of the products of the process and appears in the waste gases normally vented to the atmosphere. Accordingly, the pollution of the atmosphere by sulfur dioxide is one of the greatest air pollution problems facing mankind today. As oil resources dwindle and a chain of nuclear plant disasters casts doubt on their safety, burning of sulfur-containing fuel (coal, natural gas and peat) is expected to increase.
Removal of gaseous SO2 in coal-burning power plants is routinely performed by reacting the waste gas (flue gas) with limestone (principally CaCO3) or hydrated lime, mainly Ca(OH)2, dispersed in water or utilized in solid form. These techniques are moderately efficient since about 0.05% SO2 still remains in the gas. Moreover, such reactions require a large consumption of reagents in parallel absorption of CO2 and are rather expensive. Furthermore, as an additional product, huge amount of solid waste (usually CaSO4) is produced, which is not utilized.
Methods for the removing sulfur dioxide from a flue gas by absorption in a molten salt mixture containing alkali metal carbonate-containing solution are described in U.S. Pat. Nos. 3,438,727 and 3,438,728. These methods include the following general steps: (1) absorption of the sulfur dioxide in a molten salt mixture containing alkali metal carbonate to form alkali metal sulfite, (2) reduction of the alkali 5 metal sulfite by treatment with hydrogen, carbon monoxide, or a mixture thereof to form alkali metal sulfide, and (3) treatment of the alkali metal sulfide with a gaseous mixture containing steam and carbon dioxide to form hydrogen sulfide and regenerate the alkali metal carbonate absorbent. In operation, the molten carbonate salt is sprayed into desulfurized 10 flue gas which flows into a demister that serves to remove entrained salt-containing droplets from the flue gas which is then passed in an absorber tower and then through a plurality of heat exchangers from which it is discharged to the atmosphere.
The molten sulfite-containing carbonate resulting from the reaction between the molten carbonate spray and the flue gas is collected in a dished-bottom heated sump of absorber. The sulfite-carbonate mixture is then pumped from sump of absorber to a heat exchanger. The mixture leaves heat exchanger by way of a conduit and passes through a heater, which is optionally utilized for further increasing the temperature of the mixture. The mixture leaves heater through a conduit where it is fed into a spray distributor in a reducer unit.